André was correct in claiming that phaneroscopy must involve a
transition from experience in the phaneron to something that can be
interpreted by ordinary common sense.  But he made a mistake in
claiming
that it could not be mathematical.

See below for
five quotations by CSP, which make the following points.

1.
>From the first quotation, a diagram is an "icon, which exhibits a
similarity or analogy to the subject of discourse."

2.
"we construct an icon of our hypothetical state of things and
proceed
to observe it... We not only have to select the features of
the diagram
which it will be pertinent to pay attention to, but it is
also of great
importance to return again and again to certain
features."

3. A diagram may be "a concrete, but
possibly changing, mental image of
such a thing as it
represents."

4. "We form in the imagination some sort
of diagrammatic, that is,
iconic, representation of the facts, as
skeletonized as possible."

5. "Diagrammatic reasoning
is the only really fertile reasoning."

For more quotations
by Peirce, search CP for the 538 instances of
'diagram' with or
without various suffixes.  They show that diagrams are
"iconic", they are fundamental for "necessary
reasoning" in mathematics,
and they can be understood by anybody
that André calls "the rest of us".

John

------------------------------

1. there are three kinds of
signs which are all indispensable in all
reasoning; the first is the
diagrammatic sign or icon, which exhibits a
similarity or analogy to
the subject of discourse.  [second is index;
third is symbol] (CP
1.369)

2. All necessary reasoning without exception is
diagrammatic.  That is,
we construct an icon of our hypothetical
state of things and proceed to
observe it.  This observation leads us
to suspect that something is
true, which we may or may not be able to
formulate with precision, and
we proceed to inquire whether it is
true or not.  For this purpose it is
necessary to form a plan of
investigation, and this is the most
difficult part of the whole
operation.  We not only have to select the
features of the diagram
which it will be pertinent to pay attention to,
but it is also of
great importance to return again and again to certain
features.  (EP
2:212, 1903)

3. The word diagram is here used in the peculiar
sense of a concrete,
but possibly changing, mental image of such a
thing as it represents.  A
drawing or model may be employed to aid
the imagination; but the
essential thing to be performed is the act
of imagining.  Mathematical
diagrams are of two kinds; 1st, the
geometrical, which are composed of
lines (for even the image of a
body having a curved surface without
edges, what is mainly seen by
the mind’s eye as it is turned about, is
its generating lines, such
as its varying outline); and 2nd, the
algebraical, which are arrays
of letters and other characters whose
interrela-tions are represented
partly by their arrangement and partly
by repetitions.  If these
change, it is by instantaneous metamorphosis.
(NEM 4:219)

4, We form in the imagination some sort of diagrammatic, that is,
iconic, representation of the facts, as skeletonized as possible. 
The
impression of the present writer is that with ordinary persons
this is
always a visual image, or mixed visual and muscular...  This
diagram,
which has been constructed to represent intuitively or
semi-intuitively
the same relations which are abstractly expressed in
the premisses, is
then observed, and a hypothesis suggests itself
that there is a certain
relation between some of its parts -- or
perhaps this hypothesis had
already been suggested.  In order to test
this, various experiments are
made upon the diagram, which is changed
in various ways.  (CP 2.778)

5. Diagrammatic reasoning is the
only really fertile reasoning.  If
logicians would only embrace this
method, we should no longer see
attempts to base their science on the
fragile foundations of metaphysics
or a psychology not based on
logical theory.  (CP 4.571)


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